HMU is short for hit me up. Hit me up means contact me later if you need me as I am available for whatever.
How to use hmu in a sentence?
If you want to go out today hmu, I am bored.
When to use hmu?
You can use it while chatting on your mobile phone to keep the sentence short but convey more information.
Is hmu flirty?
No, it is not. It simply means keep me in mind and contact me if you do a certain thing which I am interested in. But, it could certainly be used in a flirty way.